Instead of defining a free function print
as the callback handler, as we did in the earlier tutorial programs, we now
define a class called printer.

classprinter{public:

The constructor of this class will take a reference to the io_service object
and use it when initialising the timer_
member. The counter used to shut down the program is now also a member of
the class.

The boost::bind() function works just as well with class member functions
as with free functions. Since all non-static class member functions have
an implicit this parameter,
we need to bind this to the
function. As in tutorial Timer.3, boost::bind() converts our callback handler
(now a member function) into a function object that can be invoked as though
it has the signature void(constboost::system::error_code&).

You will note that the boost::asio::placeholders::error placeholder is not
specified here, as the print
member function does not accept an error object as a parameter.

timer_.async_wait(boost::bind(&printer::print,this));}

In the class destructor we will print out the final value of the counter.

~printer(){std::cout<<"Final count is "<<count_<<"\n";}

The print member function
is very similar to the print
function from tutorial Timer.3, except that it now operates on the class
data members instead of having the timer and counter passed in as parameters.